One of the principal insulation materials for conductors used in telephone communication systems is a pulpous material which tends to absorb moisture to which a cable may be exposed, thereby avoiding degradation of transmission signals. Water which enters a pulp-insulated multipair cable causes the pulp to swell, which localizes the water at a fault point that is detectable by routine tests. Pulp-insulated cable advantageously is low in cost, provides more conductors for a given cable diameter than other kinds of cable, and has a continuing availability as opposed to sources for a petroleum derivative from which plastic insulating materials are made.
Pulp-insulated conductors are generally produced in a continuous process in which many conductors, often as many as sixty, are passed through an electrolytic cleaner, coated with a wet pulp layer, and drawn through a drying oven in order to produce a pulp-insulation cover having a final moisture content in the range of 7 to 10% on each of the conductors. A detailed description of a pulp-insulating process can be had by referring, for example, to an article "Manufacturing Pulp Cable," on pages 86-94 of the July-October 1971 issue of The Western Electric Engineer.
One of the problems in pulp-insulated conductors is the occurrence of uninsulated areas along the conductors, particularly in the final cable structure. These may occur either because of a lack of adherence of the pulp to the conductor during insulating or because of the abuse to which the insulation is subjected in steps of a cable-making process subsequent to insulating. Conductors which have a predetermined number of such defects occupy a part of and increase the size of the cable cross-section without contributing to its utility since they are unuseable for telecommunications. Unfortunately, an increased cross-section requires additional plastic jacketing material and underground duct capacity without any offsetting benefit.
Another problem relates to the strength characteristics of pulp insulation and the effects of these on the electrical properties of pulp-insulated conductors. When two insulated conductors are associated together to form a twisted pair, their centers are separated by a distance which has an inversely proportional effect on mutual capacitance. Because the crush resistance of conventional pulp-insulation having a residual moisture content is relatively low, one or both of the conductors may have its insulation deformed when subjected to the rigors of other manufacturing processes such as, for example, twisting. This generally causes the distance between conductor centers to be decreased with an accompanying undesirable increase in mutual capacitance. While this problem could be overcome by reducing the residual moisture content, the resulting pulp insulation would have unacceptable flexibility and endurance characteristics.
These problems have been overcome by a process which is disclosed in commonly assigned application Ser. No. 951,808 filed Oct. 16, 1978 now U.S. Pat. No. 4,218,285 which issued on Aug. 19, 1980 in the names of H. E. Durr and James G. Wright, Jr. The pulp is treated to optimize its resistance to deformation in subsequent processing and during installation to preserve acceptable mutual capacitance properties, without causing the pulp to become brittle and impair the integrity of the insulative cover. In that process an electrical conductor is enclosed in a coating which is capable of being treated so that it is substantially insoluble in water, and which when covered with pulpous material is capable of being further treated to create an adhesive bond between the pulpous material and the conductor. The coated conductor is treated to render the coating substantially insoluble in water after which it is enclosed in a pulpous material having a relatively high moisture content. The coating and the pulpous material are further treated to reduce substantially the moisture content of the pulpous material and to tackify the coating which cause the crush resistance to be increased and the pulpous material to adhere to the conductor to form an insulative cover having substantial integrity.
The material which is coated on the conductors prior to the application of pulpous material is an acrylic latex material which tends to harden as it engages a surface. Further, as additional material runs over existing material, it tends to build up since it tends to adhere to itself more rapidly than to surfaces. This behavior by the latex material causes problems during its application to the conductors. Desirably, an apparatus for its application to a plurality of conductors is such that the area of surface contact with the material is controlled to minimize build up.
In the prior art, apparatus which sprays material on moving conductors is known but sprays are not easily controllable, especially for small gauge conductors. Rollers have been used to apply a coating from a bath to a conductor which is advanced thereover at a tangent point of engagement. However, neither of these techniques is likely to result in a uniform thickness coating on each of a plurality of conductors being advanced along parallel paths.
An apparatus shown in U.S. Pat. No. 2,943,598 insulates a wire by moving a wire in a plurality of passes along inclined paths through a notched weir which is mounted on each side of an oven. While multiple passes are necessary to provide an insulation cover on a conductor instead of causing undue sag of the coating material in a single pass between an applicator and an oven, such an arrangement is not feasible in a pulp insulating line, which includes other equipment, for the simultaneous coating of a plurality of conductors with a precise thickness layer which will not adversely affect the dielectric properties of the pulp-insulated conductors.
The prior art also seemingly lacks a coating apparatus which is capable of simultaneously providing each of a plurality of conductors with a uniform thickness layer while avoiding undue build-up of coating material on the apparatus which otherwise would impair the efficiency of the apparatus.